Boston’s artificial land vulnerable to climate change, harder to protect

As climate change worsens and sea levels climb, the huge share of artificial land here is not just an interesting factoid of history, but a significant construction challenge for the dozens of coastal resilience projects, including Ryan Playground, that will need to be built for the coming decades. Artificial land is generally lower in elevation, and it cannot always hold as much weight as natural land without subsiding, Kunkel said.

About a sixth of the city was artificially constructed; the proportion of natural land to made land is even higher in coastal neighborhoods like East Boston, Charlestown, and downtown.

“As far as I know, Boston has more [made land] than any [city] in the United States, and I’m pretty sure more than any in North America,” said Nancy Seasholes, a historian and historical archaeologist who wrote a history of landmaking in Boston.

That means the artificial land is both more vulnerable to sea level rise and more difficult to protect from flooding with heavy sea walls. But without those projects as climate change worsens, Boston’s map of flooding risks during severe storms could look a lot like the map of Boston’s shoreline hundreds of years ago, according to a state model that estimates future flood plains.

For two centuries starting in the late 1700s, artificial land was constructed throughout Boston on top of tidal flats with almost anything the engineers of the day could think of: mostly dirt from nearby hills, but also excavated mud, garbage, and coal ash.

High points throughout Boston were removed, including Beacon Hill. At its summit, Beacon Hill once towered 60 feet higher than today, according to one calculation. Across Boston, materials were piled up to create new land, usually behind granite sea walls, until the ground reached a few feet above high tide.

“They thought to fill it above the level of high tide was adequate, and it was — at the time,” Seasholes said.

Now, however, climate change is pushing Boston’s high tide higher.

As the planet has warmed, glaciers and sea ice have melted, adding more water to the seas. Rising ocean temperatures have also expanded the volume of water. The sea level in Boston has been increasing a couple inches per decade over the past century, but now the pace of increase is quickly accelerating. The city is likely to have a waterline that’s a foot higher than it was in 2000 by mid-century, scientists estimate. The seas are rising faster here than the global average.

Longer-term projections are more dire. Depending on global greenhouse gas emissions over the coming decades, Boston could see between 2 and 4 feet of sea level rise as soon as 2070. (Humanity still has time to reach net zero global emissions by 2050; if we do, sea level rise would likely be less.)

As more coastal protections — from sea walls to elevated land — are built across the city to shield neighborhoods from worsening floods during storms and at high tide, artificial land poses tricky complications.

At the Ryan Playground, engineers are trying to figure out how to ensure the land is strong enough to hold the weight of the berm, which will be disguised as a sloping landscaped area, playground, and river walk.

“We’re very concerned about settlement over time,” said Kunkel, the project’s manager, “as well as the violence of storm events eroding out the walls and compromising them.”

Similar problems have cropped up on other projects he’s done in the city, he said.

Boston was constructed from a geologic formation called a tombolo, or an island that is connected to the mainland by narrow strips of land. For the first 150 years that Boston existed, the tombolo, now downtown Boston, was connected to the mainland by only one road. Over subsequent centuries, that original peninsula doubled in size.

As Boston’s population grew, land developers created more real estate throughout the 17th and 18th centuries by laying down timbers in a crosshair pattern and dumping fill — at that time, usually garbage — on top of it, according to Seasholes, the historian.

In the 19th century, techniques changed; developers began to build stone sea walls and fill behind that point until it was above high tide. Trash was no longer used because it was suspected as a cause for disease (later debunked). Instead, land builders used gravel, mud from tidal flats, or excavated dirt. That’s why land developers chopped off the top of Beacon Hill and the hills throughout Charlestown, Seasholes said.

Landowners began selling dirt excavated from the sides of Beacon Hill beginning in about 1810, according to Seasholes’ research. In 1811, the town sold Beacon Hill to pay down debt, opening the door for landowners to cut down the entirety of the hill and sell the dirt; most of it was likely used for Mill Pond (an area now called Bulfinch Triangle, approximately between Causeway Street and Haymarket Square). A Revolutionary War monument column that had topped the hill was removed.

Later, when the railroads came, developers began shipping in material from farther away to build neighborhoods including parts of the South End. Back Bay, the largest artificially created neighborhood in the city, was built with materials this way beginning in the 1850s.

An aerial view of filled-in land at the controversial marina development near Tenean Beach in the Dorchester neighborhood of Boston on Sept. 23, 1969. Sam Masotta/Globe Staff

Today, different neighborhoods have unique engineering challenges due to that history of shifting techniques to create land.

In Back Bay, for example, droughts — not floods — are the more pressing structural threat because the area was built on wood pilings that need to be saturated by groundwater or risk decay.

Boston Logan International Airport, meanwhile, was built at a slightly higher elevation when that land was constructed, which occurred mostly in the 1940s due to the growth of the aviation industry during World War II. The airport, therefore, is much less susceptible to coastal flooding than nearby residential East Boston, which is one of the most flood vulnerable neighborhoods in the city.

A truckload of rocks was hauled out on the dike being constructed in Boston Harbor as part of a runway extension over the Bird Island Flats at Logan Airport on April 6, 1970. Bill Ryerson/Globe Staff

The airport was an exception. In general, experts said, no more land was built than was necessary at the time. Adding another few inches or feet to the elevation cost more time, materials, and money — the same constraints that many coastal resilience projects face today.

“The goal was to get it dry, get it flat, and then sell it,” said Garrett Dash Nelson, a historical geographer at the Norman B. Leventhal Map & Education Center at the Boston Public Library. “They were not thinking about future proofing.”

That future proofing is happening now because much of Boston’s artificial land sits directly in the likely path that flood waters would travel during an extreme storm, particularly as the sea level rises.

That’s true at the Ryan Playground project in Charlestown, which sits adjacent to the Mystic River. If left low-lying, the park is a weak point where flood waters could infiltrate the Schrafft’s City Center, a busy area for local businesses named for the well-known candy company that was once headquartered there.

To fortify the park, hundreds of narrow stone columns will be drilled up to 65 feet deep with a boring machine. Those stone columns will help to distribute the weight of the new 7-foot berm, as well as an additional 2 feet of elevation added to much of the park. Foam glass aggregate, a porous material made from recycled glass that weighs a fraction of normal soil, will be used to fill in the land, according to Kunkel, the project manager.

The park has another complication: A plume of petroleum is trapped underneath, likely a vestige of industrial activities in the area. It’s safe to remain where it is, Kunkel said, but allowing flood water to infiltrate too deep underground could prompt that plume to move. The architects, therefore, also envision a drainage system of underground pipes.

Without that new berm in the park, 2 feet of sea level rise could bring flooding to Charlestown during an extreme storm. According to the state’s model, the path that storm water would follow closely resembles the shoreline of almost 400 years ago.

Construction on the project is expected to begin this fall and last about two years.

Erin Douglas can be reached at erin.douglas@globe.com. Follow her @erinmdouglas23.

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